As if the turquoise sea and white-powder sand isn't enough, the developers have carved lagoons and laid green turf into the limestone. It's a monument to the economic power of tourism, a city built within two decades for a single purpose--to worship the sun God.

And yet, I marvel that the real wonders of the Yucatan peninsula are beyond the fast-food restaurants and parachute rides. Most of the visitors never know that less than an hour away are secrets that link past and future in a subterranean world, only now being explored.

Our helicopter traces the coastline to the spectacular Mayan ruins of Tulum, then turns inland over the green foliage that stretches hundreds of miles, appearing as flat as a table top. It's deceiving. Beneath the scrubby trees is an undulating terrain of sharp-edged limestone, pitted by erosion over more than 10 millennia.

And then beneath us, the verdant carpet falls away to a deep cavern, a perfect hole in the earth. It could easily pass for the opening of another Carlsbad Caverns--except that this hole is filled with water.

The Mayans regarded them as sacred "cenotes" or fresh-water reservoirs. To modern cave divers, it's the new frontier of exploration.

What looks from a helicopter like an easy walk turns ugly on the ground. The trail coils over twisted vines and loose rocks. The deadly fer de lance snake calls this area home. If you miss the snake, you just might run into the chi chin tree that can leave you with red welts worse than poison ivy.

With the help of a family of Mayans, Mike Madden, owner of CEDAM dive centers at Puerto Aventuras about 40 minutes south of Cancun, packs his heavy scuba air canisters on the backs of mules and leads tourists to a cenote he's named Nohoch Nah Chich, a Mayan word for "giant birdhouse." But his primary mission begins after the tourists have left.

Along with other dedicated divers, his team has already established Nohoch as the longest underwater cave system in the world. But he isn't satisfied. He's obsessed with the final achievement. Mike Madden is determined to head the first team that connects the 27-mile cave system with the Caribbean Sea, several miles away. It will be strong evidence that Yucatan's sacred cenotes are really underground rivers carrying rain water to the ocean.

My "New Explorers" camera team is ready to go along on one of the final attempts. We join Madden in the last stage of his obsession. He started this project eight years ago.

This is far too dangerous for my diving skills. I'll only go a few thousand feet, always under the watchful eye of my dive master, Peter Butt of Florida. Cave diving is the most dangerous sport in the world. More than 360 people have drowned in North American caves since 1960. Some were caught in falling rocks but most reached beyond the limits of their experience. The National Speleological Society Cave Diving Section recommends that a minimum of 100 dives is necessary to attempt the sport. It requires a cool head not easily disturbed by stressful situations and a thorough knowledge of the specialized dive gear necessary to live for up to five hours in an environment more alien than space.

Madden's team will use the side-mount technique of placing their air tanks along their bodies instead of their backs so they can slide easily through narrow passages. They'll cache extra tanks along the way for the return trip. All systems must be redundant. A minimum of three lights is required. Most divers carry four.

"New Explorers" cameraman Wes Skiles reminds me that "when you run out of air, survival is measured not in seconds, but in breaths."

The water is cold, even in the steamy tropical heat of Mexico. Motmot birds swoop into their rock nests above us as we slip into the water beneath a rock overhang.

And then the most dramatic moment of the experience comes with our first look underwater. An oxygen-filled world of daylight becomes the Mayans' "other" world where they believed their gods reside.

My first thought was that they were dead right.

The cave floor drops sharply to a depth of 40 feet. Giant flowstones line the sides like columns in an Egyptian temple. Stalactites hang from above in all shapes and sizes, some layered like an upside-down ice cream cone, others in dagger shapes.

It reminds me of Kentucky's Mammoth Cave, only filled with water.

We release air from our buoyancy vests and glide silently to the cave floor where the columns tower over us, glistening in our lights.

Before we began, I asked team member Jeffrey Haupt how good the visibility was in the cave. He said, "As far as your light will reach."